With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Men's Work Inch Clad Boot Brown Iron LTT Rocky Eight alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Eight Boot Clad Work Brown LTT Iron Rocky Inch Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Pump Women's Mid Sandals Beige Platform PAIRS Heel Michelle DREAM Zw56qYP

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end